Artificial photosynthetic model systems using light-induced electron transfer reactions in catalytic and biocatalytic assemblies. In: Mattay J, editor. Photoinduced Electron Transfer III. Berlin: Springer Berlin Heidelberg; 1991. pp. 153-218. [DOI: 10.1007/3-540-53257-9_4]

Synthesis and solvent-dependent photochromic reactions of porphyrin-spiropyran hybrid compounds

Porphyrin(Por)-spiropyran(SP) hybrid compounds, including Por-SP dyad, Por-SP2 triad, and Por-SP4 pentad, were prepared and characterized by (1)H NMR, MALDI-TOF MS and UV-Vis spectroscopies. Upon 350 nm UV irradiation of Por-SPn (n=1, 2, 4) in dichloromethane, unusual red-shifted absorption spectra were observed with the colour change from pink into green. Probably due to the protonation of core nitrogens in porphyrin ring, their absorption maxima in dichloromethane were shifted from 418 (Soret band), 515, 550, 590, 645 (four Q bands) nm into 450 and 665 nm. Also, fluorescence maxima were also shifted from 650 and 715 nm to 692 nm. In the other hands, upon irradiation with 350nm UV light in THF, the colour changed from pink into violet and absorption band at 590 nm increased and the fluorescence spectra showed the decrease of 650 and 715 nm bands and increase of 600-640 nm band, due to the normal ring-opening reaction of spiropyran moiety into merocyanine. In the dark, original absorption and fluorescence spectra were recovered very slowly in dichloromethane, but quickly in THF. The reversible photochromic reactions of Por-SPn (n=1, 2, 4) in dichloromethane and THF were investigated by observing absorption and fluorescence spectral changes during UV irradiation or standing in the dark.

Photochemical conversion of solar energy

Energy is the most important issue of the 21st century. About 85% of our energy comes from fossil fuels, a finite resource unevenly distributed beneath the Earth's surface. Reserves of fossil fuels are progressively decreasing, and their continued use produces harmful effects such as pollution that threatens human health and greenhouse gases associated with global warming. Prompt global action to solve the energy crisis is therefore needed. To pursue such an action, we are urged to save energy and to use energy in more efficient ways, but we are also forced to find alternative energy sources, the most convenient of which is solar energy for several reasons. The sun continuously provides the Earth with a huge amount of energy, fairly distributed all over the world. Its enormous potential as a clean, abundant, and economical energy source, however, cannot be exploited unless it is converted into useful forms of energy. This Review starts with a brief description of the mechanism at the basis of the natural photosynthesis and, then, reports the results obtained so far in the field of photochemical conversion of solar energy. The "grand challenge" for chemists is to find a convenient means for artificial conversion of solar energy into fuels. If chemists succeed to create an artificial photosynthetic process, "... life and civilization will continue as long as the sun shines!", as the Italian scientist Giacomo Ciamician forecast almost one hundred years ago.

An approach towards artificial quinone pools by use of photo- and redox-active dendritic molecules

Molecules with one porphyrin and multiple quinone groups are described. The molecules are based on dendritic frameworks, with the quinone groups attached at the "internal" positions and the porphyrin attached at the focal point, leading to a characteristic layer architecture. When irradiated with visible light in the presence of 4-tert-butylthiophenol, the quinones were converted to quinols. Such a behavior mimics the function of quinone pools in photosynthesis.

Catalytic Photooxidation of 4-Methoxybenzyl Alcohol with a Flavin-Zinc(II)-Cyclen Complex

Flavin-zinc(II)-cyclen 10 contains a covalently linked substrate binding site (zinc(II)-cyclen) and a chromophore unit (flavin). Upon irradiation, compound 10 effectively oxidizes 4-methoxybenzyl alcohol (11-OCH3) to the corresponding benzaldehyde both in water and in acetonitrile. In the presence of air, the reduced flavin 10-H2 is reoxidized, and so catalytic amounts of 10 are sufficient for alcohol conversion. The mechanism of oxidation is based on photoinduced electron transfer from the coordinated benzyl alcohol to the flavin chromophore. This intramolecular process provides a much higher photooxidation efficiency, with quantum yields 30 times those of the comparable intermolecular process with a flavin chromophore without a binding site. For the reaction in buffered aqueous solution a quantum yield of Phi = 0.4 is observed. The turnover number in acetonitrile is increased (up to 20) by high benzyl alcohol concentrations. The results show that the covalent combination of a chromophore and a suitable binding site may lead to photomediators more efficient than classical sensitizer molecules.

Glutamate synthesis via photoreduction of NADP+ by photostable chlorophyllide coupled with polyethylene-glycol

Chlorophyllide a was coupled with alpha-(3-aminopropyl)-omega-methoxypoly(oxyethylene) (PEG-NH2) to form a PEG-chlorophyllide conjugate through an acid-amide bond. The conjugate catalyzed the reduction of methylviologen in the presence of 2-mercaptoethanol. It also catalyzed the photoreduction of NADP+ or NAD+ in the presence of ascorbate as an electron donor and ferredoxin-NADP+ reductase as the coupling enzyme. Utilizing the reducing power of NADPH generated by PEG-chlorophyllide conjugate under illumination, glutamate was synthesized from 2-oxoglutarate and NH4+ in the presence of glutamate dehydrogenase. PEG-chlorophyllide conjugate was quite stable toward light illumination compared with chlorophyll a. The increase in the molecular weight of PEG in the PEG-chlorophyllide conjugates was accompanied by the enhancement of photostability of the conjugate and also by the increased solubility in the aqueous solution.

Hydrogen gas evolution and carbon dioxide fixation with visible light by chlorophyllin coupled with polyethylene glycol

Chlorophyllin a was conjugated with alpha-(3-aminopropyl)-omega-methoxypoly(oxyethylene), PEG-NH(2), to form the PEG-chlorophyllin conjugate through acid-amide bonds. The PEG-chlorophyllin conjugate was stable toward light illumination under anaerobic condition in comparison with chlorophyllin a. The conjugate catalyzed the reduction of methyl viologen in the presence of 2-mercaptoethanol and the evolution of hydrogen gas in the presence of methyl viologen (an electron carrier), 2-mercaptoethanol (an electron donor) and hydrogenase (Scheme 1). Furthermore, the PEG-chlorophyllin conjugate catalyzed the photoreduction of NADP(+) or NAD(+) in the presence of ascorbate as an electron donor and ferredoxin-NADP(+) reductase as the coupling enzyme. Utilizing the reducing power of NADPH generated by the PEG-chlorophyllin conjugate under the illumination, CO(2) fixation was accomplished by the synthesis of malate (C(4)) from pyruvate (C(3)) and CO(2) in the presence of malic enzyme (Scheme 2). These reactions mentioned above did never proceed in dark or without each enzyme.

Molecular Mechanics (MM3) Parameters for Ruthenium(II)-Polypyridyl Complexes

We have developed molecular mechanics parameters for Ru(II)-polypyridyl coordination compounds with the MM3 force field in MacroModel. X-ray structures, together with a B3LYP frequency calculation on a model system, have been utilized in the parametrization. The performance of the force field and the quality of each parameter is analyzed. A clear qualitative correlation have been found between coordination geometry and emission properties for the ruthenium polypyridyl complexes examined in this paper.

Synthesis, Structure, and Photophysical Properties of Novel Ruthenium(II) Carboxypyridine Type Complexes

A series of Ru(II) compounds and salts have been synthesized: [Ru(6-carboxylato-bpy)(2)] (5), [Ru(6-carboxylato-bpy)(tpy)]PF(6) (9), [Ru(tpy)(2)](PF(6))(2) (8), and [Ru(bpy)(2)(Pic)]PF(6) (11), where 6-carboxy-bpy (1) = 6-carboxy-2,2'-bipyridine, tpy (2) = 2,2':6',2"-terpyridine, and Pic = 2-carboxylatopyridine. The compounds have been characterized by NMR, electrospray mass spectrometry (ESI-MS), cyclic voltammetry, absorption and emission spectroscopy (at 100, 140, and 298 K), and single-crystal X-ray diffraction (complex 5). Complex 5 crystallizes in the monoclinic system, space group P2(1)/n, formula RuC(22)H(14)N(4)O(4).C(2)H(5)OH, with a = 11.088(3) Å, b = 11.226(3) Å, c = 35.283(9) Å, beta = 91.41(2) degrees, and Z = 8. A linear dependence on the number of coordinated carboxylato groups and the electrochemical redox potentials was found, ca. 0.4 V lower reduction potential for the oxidation step (Ru(II/III)) per carboxylate group. Also, to the best of our knowledge, these are the first examples (9, 11) of mononuclear Ru(II) complexes containing a carboxypyridine-ruthenium moiety displaying any luminescence emission.

Stereochemical Control of Donor and Acceptor Groups in a Monomeric Chromophore-Quencher Complex of Ruthenium(II)

The chromophore-quencher complex [Ru(Me(2)bpy)(bpy-MV(2+))(bpy-PTZ)](4+), containing one donor (phenothiazine, PTZ) and one acceptor (methyl viologen, MV(2+)) functionality, has been synthesized and separated into its four geometric isomers. This was achieved through the intermediacy of [Ru(Me(2)bpy)(bpy-MV(2+))(py)(2)](4+), the two isomers of which were separated and each reacted stereoselectively with bpy-PTZ to produce two distinct isomer pairs of the target molecule. Cation-exchange chromatography allowed separation to realize the four forms. The isomers of the product and of the intermediates were characterized by NMR spectroscopy. This is the first example of the isolation of geometric isomers of a mononuclear system containing a single donor and single acceptor functionality.

Improving enzyme-electrode contacts by redox modification of cofactors

Efficient electron transfer of redox proteins to and from their environment is essential for the use of such proteins in biotechnological applications such as amperometric biosensors and photosynthetic biocatalysts. But most redox enzymes lack pathways that can transport an electron from their embedded redox site to an electrode or a diffusing photoexcited species. Electrical communication between redox proteins and electrode surfaces has been improved by aligning proteins on chemically modified electrodes, by attaching electron-transporting groups and by immobilizing proteins in polymer matrices tethered by redox groups. Generally these methods involve contacting the enzymes at random with electron relay units. Here we report an approach that allows site-specific positioning of electron-mediating units in redox proteins. We strip glucose oxidase of its flavin adenine dinucleotide (FAD) cofactors, modify the latter with redox-active ferrocene-containing groups, and then reconstitute the apoprotein with these modified cofactors. In this way, electrical contact between an electrode and the resulting enzyme in solution is greatly enhanced in a controlled and reproducible way.